At physiological concentrations, 17beta- estradiol enhances endothelium-dependent vasodilation when administered to postmenopausal women, but by an unknown mechanism. To assess the contribution of nitric oxide to the vascular effects of estradiol, we measured coronary epicardial and microvascular responses to intracoronary acetylcholine (range 3-300 mcg/min for 2 min) before and after intracoronary estradiol 75 ng/min for 15 min in 20 estrogen-deficient women, 16 of whom had angiographic evidence of atherosclerosis or risk factors for atherosclerosis. This testing was repeated following inhibition of nitric oxide synthesis with intracoronary NG- monomethyl-L-arginine (L-NMMA) 64 mumol/min for 5 minutes. Coronary flow was derived from intracoronary flow velocity in quantitative epicardial coronary artery diameter measurements distal to the Doppler wire. Estradiol increased acetylcholine-stimulated flow from 54+/-42% (mean+/-standard deviation)above baseline values prior to estradiol infusion to 100[unreadable]63% above baseline values (p=0.007), and potentiated microvascular dilatation as evidenced by greater decrease in coronary resistance (from 32+/-20 to 46+/-15% below baseline values, p=0.007) at a coronary sinus estradiol concentration of 470+/-192 pg/mL. Estradiol also tended to lessen the severity of acetylcholine-induced decrease in epicardial coronary artery diameter from 8+/-11 to 3+/-11% below baseline valuse (p=0.123). However, during L-NMMA infusion, estradiol no longer potentiated the effects of acetylcholine on coronary dynamics (coronary flow increased from 42+/-40 to 39+/-46% above baseline values, p=0.777; coronary resistance decreased from 23+/-21 to 19+/-30% below baseline values, p=0.553; epicardial diameter decreased from 8+/-8 to 8+/-11% below baseline values, p=0.764). We conclude that the effects of estradiol at physiological concentrations on endothelium-dependent coronary vasodilator responsiveness in postmenopausal women are mediated by enhanced bioavailability of nitric oxide. Nitric oxide may be responsible in part for the cardioprotective effects of estrogen, as suggested by observational studies, when chronically administered to postmenopausal women.